When you shoot an arrow with arrows in your off hand they are not shot first.
Steps to reproduce:
1. Get a bow (mine is infinity if this helps)
2. Put some arrows in your off hand
3. Put tipped arrows in your inventory
4. Shoot bow and the tipped arrows will shoot

Edit: It only is a problem with one of my bows. The bow is punch 2, infinity, and named. However my other bow does not and it has the same enchants plus flame and power